add a ModifyCmdFunc ExecAllocatorOption (#674)

ModifyCmdFunc allows a function to be passed which will run
on the exec.Cmd used to start the browser. This can be used to
prevent chromedp from automatically passing os signals through
to managed browsers, and to prevent browsers from being automatically
killed on program exit.

allocate.go

@@ -108,6 +108,8 @@ type ExecAllocator struct {
 	wg sync.WaitGroup
 
 	combinedOutputWriter io.Writer
+
+	modifyCmdFunc func(cmd *exec.Cmd)
 }
 
 // allocTempDir is used to group all ExecAllocator temporary user data dirs in
@@ -170,16 +172,24 @@ func (a *ExecAllocator) Allocate(ctx context.Context, opts ...BrowserOption) (*B
 			os.RemoveAll(dataDir)
 		}
 	}()
-	allocateCmdOptions(cmd)
+
+	if a.modifyCmdFunc != nil {
+		a.modifyCmdFunc(cmd)
+	} else {
+		allocateCmdOptions(cmd)
+	}
 
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {
 		return nil, err
 	}
 	cmd.Stderr = cmd.Stdout
 
-	if len(a.initEnv) > 0 {
-		cmd.Env = append(os.Environ(), a.initEnv...)
+	// Preserve environment variables set in the (lowest priority) existing
+	// environment, OverrideCmdFunc(), and Env (highest priority)
+	if len(a.initEnv) > 0 || len(cmd.Env) > 0 {
+		cmd.Env = append(os.Environ(), cmd.Env...)
+		cmd.Env = append(cmd.Env, a.initEnv...)
 	}
 
 	// We must start the cmd before calling cmd.Wait, as otherwise the two
@@ -391,6 +401,16 @@ func Env(vars ...string) ExecAllocatorOption {
 	}
 }
 
+// ModifyCmdFunc allows for running an arbitrary function on the
+// browser exec.Cmd object. This overrides the default version
+// of the command which sends SIGKILL to any open browsers when
+// the Go program exits.
+func ModifyCmdFunc(f func(cmd *exec.Cmd)) ExecAllocatorOption {
+	return func(a *ExecAllocator) {
+		a.modifyCmdFunc = f
+	}
+}
+
 // UserDataDir is the command line option to set the user data dir.
 //
 // Note: set this option to manually set the profile directory used by Chrome.

allocate_test.go

@@ -348,3 +348,31 @@ func TestWithBrowserOptionAlreadyAllocated(t *testing.T) {
 		WithLogf(func(format string, args ...interface{}) {}),
 	)
 }
+
+func TestModifyCmdFunc(t *testing.T) {
+	t.Parallel()
+
+	tz := "Atlantic/Reykjavik"
+	allocCtx, cancel := NewExecAllocator(context.Background(),
+		append([]ExecAllocatorOption{
+			ModifyCmdFunc(func(cmd *exec.Cmd) {
+				cmd.Env = append(cmd.Env, "TZ=" + tz)
+			}),
+		}, allocOpts...)...)
+	defer cancel()
+
+	ctx, cancel := NewContext(allocCtx)
+	defer cancel()
+
+	var ret string
+	if err := Run(ctx,
+		Evaluate(`Intl.DateTimeFormat().resolvedOptions().timeZone`, &ret),
+	); err != nil {
+		t.Fatal(err)
+	}
+
+	if ret != tz {
+		t.Fatalf("got %s, want %s", ret, tz)
+	}
+}
+